This invention relates generally to food freezers and refrigerators and, more particularly, to the cabinets of such appliances in which either the evaporator or condenser coils, or both, are formed by attaching tubing directly to a sheet metal panel forming a portion of the cabinet.
In refrigerators and food freezers, the condenser unit is generally constructed in one of two ways. One way which is used when the refrigeration loads are quite high is to use an external condenser unit, generally mounted on the rear face of the cabinet, in which the tubing through which the refrigerant flows is connected to other metallic members, such as a grid of metal wires, and with fan-assisted air flow allows the heat to be radiated to the atmosphere. The other type of construction places the tubing directly on the inside of one or more of the exterior panels of the cabinet so that the exterior cabinet panel itself is heated above room temperature and the entire surface of the panel then acts as a radiator to remove the necessary heat from the system. In certain cases, such as chest-type food freezers with the opening in the top, a similar approach has been used also for the evaporator tubing, which is then attached to the exterior of the interior lining so that the entire interior wall becomes cold to absorb heat from the contents.
In the type of construction where the tubing is attached to a cabinet wall, there are a number of advantages and disadvantages compared to a separate condenser unit. When the tubing is attached directly to the interior side of the exterior walls, the extra expense of the nontubing portion of the separate condenser construction is eliminated. The absence of the separate condenser unit reduces the amount of cleaning and maintenance required. When the exterior panels are the heat radiating surfaces, they have a pleasant warmth to the touch, and by placing the tubing adjacent the door opening, where condensation from the atmosphere tends to collect, the need for separate heaters in this area is avoided. Thus, it has been recognized that by attaching the condenser tubing to the interior of the external cabinet wall, considerable cost savings are achieved, and, particularly if the attachment can be done in a low-cost and efficient manner, it does not destroy the exterior appearance of the panels and provides good heat conducting contact between the tubing and the panels.
Such arrangements have been shown in connection with freezers and refrigerators in a number of prior art patents, such as U.S. Pat. No. 2,622,753, which shows a chest-type food freezer in which the condenser coils are secured to the inner side of all four exterior wall panels and evaporator tubing is likewise attached to the exterior sides of the liner member with the insulation for the cabinet separating the evaporator and condenser tubing. A number of ways have been proposed for attaching the tubing to the panel, which particularly presents a problem because the metal of the panel is generally quite thin. This is especially true with foam insulation, where, because of the rigidity of the foam, the panels can be made substantially thinner than when a fiber glass type insulation is used which adds no stiffness to the cabinet structure.
One method that has long been used to attach the tubing is by means of metal straps that, in effect, clamp the tubing in place to the inside of the panel and, in turn, are welded to a surface of the panel, as shown in U.S. Pat. Nos. 1,987,422 and 3,150,796. This method has been successful because the straps or clamps can be held to the panel with a plurality of small welds that do not cause any substantial distortion or discoloration on the external surface of the panel, particularly when the straps or clamps are made of sheet steel substantially thinner than the material used for the cabinet wall. However, the placement of the tubing on the metal panel, the subsequent placing of the straps over the tubing, and the production of a number of individual spot welds still tend to result in a rather labor-intensive method of manufacturing, and if errors are made in the placement of the straps or the location of the welds, it may be difficult or impossible to maintain the tubing in proper contact with the metal panel for maximum heat transfer. This is particularly true with tubing which is assembled to a plurality of panels when they are in a flat or planar configuration, and after the tubing is attached they are bent to form a number of intersecting sides or top portions of the panel, since, on the bending of the sheet with the tubing attached, there may be some tendency for the tubing to pull away from the sheet in certain locations if the welds have not been properly made.
One approach that has been used to avoid this is to use a brazing or soldering type operation as shown in U.S. Pat. Nos. 2,691,813; 2,386,889; and 2,276,811. While each of these arrangements can produce a highly satisfactory article with excellent heat conducting contact between the tubing and the metal sheet, they are very expensive to make and require substantial capital investment in brazing ovens and other equipment, and lower temperature approaches such as soldering are simply not applicable when both the tubing and the panels are made of steel for purposes of maintaining cost at a minimum.
More recently, other arrangements have been proposed to use nonmetallic bonding or gluing materials, as shown in U.S. Pat. Nos. 2,795,035; 4,024,620; 3,904,721; and 3,966,283. Again, these approaches have not been competitive costwise with the use of metallic straps or clamps, in part because the adhesive or other material requires time to harden and reach full strength. Further, its presence directly between the tubing and the sheets tends to reduce heat conductance from the tubing to the metal panel.
Another approach that has been used has been to weld the tubing directly to the metal sheet by a form of electrical resistance welding. For example, evaporator tubing has been continuously welded to the outside of liners for chest-type food freezers, but such arrangement requires that the tubing be attached to the liner before it is completely formed in shape and requires a progressive weld as the tubing contacts the liner, which makes it extremely difficult to control and generally results in overheating the thin sheet metal panel portion to the point where the other or exposed side is deformed, leaving visible weld marks on the surface. Furthermore, particularly in the case of condenser tubing attached to several external panels of the cabinet such as the top and two adjacent sides, the most efficient arrangement is to apply the tubes in a serpentine configuration and any form of continuous welding requires extremely complex equipment and suffers from the same problem.
As a traveling welding head moves along the tubing and the adjacent panel, the exact points of conductance are hard to control, so that the welding tends to be nonuniform and overheating or burning occurs at various points, thereby damaging the exterior finish of the panels.
To avoid this problem, it has been proposed that such welding be done only at isolated points along the tubing. Such an arrangement has been proposed in U.S. Pat. No. 3,104,312, in which the tubing is formed with annular bulges or ridges extending around the periphery, and the welding is done only at those points. However, the forming of the annular rings or projections is extremely expensive, and must generally be done while the tubing is straight and before it is formed into the configuration for attachment to the panel. A similar approach is also shown in U.S. Pat. No. 3,193,659, wherein the projections are formed not by expanding the wall of the tubing but by deforming the portions between spaced projections which remain from the undeformed portion of the tubing. Again, the equipment to accomplish this would tend to prove quite expensive and inflexible for different arrangements.
Still another approach as disclosed in U.S. Pat. No. 3,926,358 has been to use ultrasonic welding at spaced locations instead of electrical resistance welding, but this still requires deforming the tubing out of the normal circular cross-sectional shape, and has not been practical using the standard steel sheets and tubing which represent the lowest cost materials available.